The term "chelate" has often been misunderstood or applied in a general or catch-all fashion. A chelate is a definite structure resulting from precise requirement of synthesis. Proper conditions must be present for chelation to take place, including proper mole ratios of ligands to metal ions, pH and solubility of reactants. For chelation to occur, all components must be dissolved in solution and either be ionized or of appropriate electronic configuration in order for bonding to develop.
Chelation can be confirmed and differentiated from mixtures of components by infrared spectra through comparison of the stretching of bonds or shifting of absorption caused by bond formation.
As applied in the field of mineral nutrition, there are two allegedly "chelated" products which are commercially utilized. The first is referred to as a "metal proteinate." The American Association of Feed Control officials (AAFCO) has defined a "metal proteinate" as the product resulting from the chelation of a soluble salt with amino acids and/or partially hydrolyzed protein. Such products are referred to as the specific metal proteinate, i.e. copper proteinate, zinc proteinate, etc. This definition does not contain any requirements to assure that chelation is actually present. On the basis of the chemical reactant possibilities, there are some real reservations as to the probability of chelation occurring to any great degree. For example, the inclusion of partially hydrolyzed proteins as suitable ligands and the term "and/or" in reference to such ligands implies that products made solely from partially hydrolyzed protein and soluble salts would have the same biochemical and physiological properties as products made from combining amino acids and soluble metal salts. Such an assertion is chemically incorrect. Partially hydrolyzed protein ligands may have molecular weights in the range of thousands of daltons and any bonding between such ligands and a metal ion may be nothing more than a complex or some form of ionic attraction, i.e. the metal drawn in close proximity to carboxyl moiety of such a ligand.
While some products marketed as metal proteinates during the 1960's and 1970's were true chelates, this was prior to the adoption of the AAFCO definition. An analysis of products currently marketed as metal proteinates reveals that most, if not all, are mixtures of metal salts and hydrolyzed protein or complexes between metal salts and hydrolyzed protein. Most are impure products which are difficult to analyze and are not consistent in protein make-up and/or mineral content.
The second product, referred to as an "amino acid chelate", when properly formed, is a stable, product having one or more five-membered ring formed by reaction between the carboxyl oxygen, and the .alpha.-amino group of an .alpha.-amino acid with the metal ion. Such a five-membered ring is defined by the metal atom, the carboxyl oxygen, the carbonyl carbon, the .alpha.-carbon and the .alpha.-amino nitrogen and is generally represented by the following formulae. However, the actual structure will depend upon the ligand to metal mole ratio. The ligand to metal mole ratio is at least 1:1 and is preferably 2:1 but, in certain instances may be 3:1 or even 4:1. Most typically, an amino acid chelate may be represented at a ligand to metal ratio of 2:1 according to the following formula: ##STR1##
In the above formula, when R is H, the amino acid is glycine which is the simplest of the .alpha.-amino acids. However, R could be representative of any other the other twenty or so naturally occurring amino acids derived from proteins. These all have the same configuration for the positioning of the carboxyl oxygen and the .alpha.-amino nitrogen. In other words, the chelate ring is defined by the same atoms in each instance. The American Association of Feed Control Officials (AAFCO) have also issued a definition for an amino acid chelate. It is officially defined as the product resulting from the reaction of a metal ion from a soluble metal salt with amino acids with a mole ratio of one mole of metal to one to three (preferably two) moles of amino acids to form coordinate covalent bonds. The average weight of the hydrolyzed amino acids must be approximately 150 and the resulting molecular weight of the chelate must not exceed 800. The products are identified by the specific metal forming the chelate, i.e. iron amino acid chelate, copper amino acid chelate, etc.
The reason a metal atom can accept bonds over and above the oxidation state of the metal is due to the nature of chelation. In Formula I it is noted that one bond is formed from the carboxyl oxygen. The other bond is formed by the .alpha.-amino nitrogen which contributes both of the electrons used in the bonding. These electrons fill available spaces in the d-orbitals. This type of bond is known as a dative bond or a coordinate covalent bond and is common in chelation. Thus, a metal ion with a normal valency of +2 can be bonded by four bonds when fully chelated. When chelated in the manner described the divalent metal ion is completely satisfied by the bonding electrons and the charge on the metal atom (as well as on the overall molecule) is zero. This neutrality contributes to the bioavailability of metal amino acid chelates.
Amino acid chelates can also be formed using peptide ligands instead of single amino acids. These will usually be in the form of dipeptides or tripeptides because larger ligands would have a molecular weight which would be too great for direct assimilation of the chelate formed. Generally, peptide ligands will be derived by the hydrolysis of protein. However, peptides prepared by conventional synthetic techniques or genetic engineering can also be used. When a ligand is a di- or tripeptide a radical of the formula [C(O)CHRNH].sub.e H will replace one of the hydrogens attached to the nitrogen atom in Formula I. R, as defined in Formula I, can be H, or the residue of any other naturally occurring amino acid and e can be an integer of 1 or 2. When e is 1 the ligand will be a dipeptide and when e is 2 the ligand will be a tripeptide.
The structure, chemistry and bioavailability of amino acid chelates is well documented in the literature, e.g. Ashmead et al-. , Chelated Mineral Nutrition, (1982), Chas. C. Thomas Publishers, Springfield, Ill.; Ashmead et al., Intestinal Absorption of Metal Ions, (1985), Chas. C. Thomas Publishers, Springfield, Ill.; Ashmead et al., Foliar Feeding of Plants with Amino Acid Chelates, (1986), Noyes Publications, Park Ridge, N.J. as well as in U.S. Pat. Nos. 4,020,158; 4,167,564; 4,216,143; 4,216,144; 4,599,152; 4,774,089; 4,830,716; 4,863,898 and others. Flavored effervescent mixtures of vitamins and amino acid chelates for administration to humans in the form of a beverage are disclosed in U.S. Pat. No. 4,725,427.
One advantage of amino acid chelates in the field of mineral nutrition is attributed to the fact that these chelates are readily absorbed in the gut and mucosal cells by means of active transport as though they were amino acids. In other words, the minerals are absorbed along with the amino acids as a single unit utilizing the amino acids as carrier molecules. Since this method of absorption does not involve the absorption sites for free metal ions, the problems of competition of ions for active sites and suppression of one nutritive mineral element by another are avoided.
The importance of vitamins and minerals in proper nutrition has long been recognized. However, it is generally thought that the absorption of vitamins and minerals is independent of each other. Even when marketing vitamin and mineral combinations, such as taught in U.S. Pat. No. 4,725,427, the vitamin and mineral ingredients are admixed and coadministered as separate entities.
There has been some discussion of some interaction between nicotinic acid, amino acids and chromium and niacinamide and cobalt in conjunction with certain amino acids and/or small peptides as they relate to components forming the glucose tolerance factor (GTF). Mertz et al., "Present Knowledge of the Role of Chromium", Federation Proc., 23(11) pp. 2275-2280 (1974) reported that, in efforts to purify the GTF, there was detected chromium, nicotinic acid, glycine, glutamic acid and cysteine. A possible structure for a dinicotinato-amino acid Cr-complex was proposed consisting of approximately 2 moles each of nicotinic acid and glycine and one mole of cysteine per chromium atom. It was readily acknowledged that the exact structure was not known. Toepfer et al., "Preparation of Chromium-Containing Material of Glucose Tolerance Factor Activity from Brewer's Yeast Extracts and by Synthesis", J. Agric. Food Chem., 25(1) pp. 162-166 (1977) report the synthesis of a chromium complex containing two moles nicotinic acid, 2 moles glycine, 1 mole glutamic acid and 1 mole of cysteine per chromium atom. Further reported is the formation of complexes of chromium with nicotinic acid alone in a presumably dinicotinato, tetaaquo configuration. This complex was stated to be unstable and rapidly lost biological activity. The formation of other complexes of chromium and nicotinic acid with other amino acids was alluded to but not specifically demonstrated. While Toepfer et al. reported GTF type biological activity it was stressed that the structure of the complexes formed was only speculative. Silio, "Process for Obtaining a New Glucose Tolerance Factor", U.S. Pat. No. 4,242,257 (1980) suggests the formation of a complex between cobalt and nicotinamide which is then reacted with the tripeptide, glutathione, to form a product having GTF activity. No structure is proposed and the reaction must be carried out stepwise with the complex between the chromium salt and nicotinamide first being formed, acidified and then reacted with glutathione.
An excellent summary of the state of the art relative to the structure and synthesis of complexes exhibiting GTF activity is found in Jensen, Synthetic GTF Chromium Material and Process Therefor, U.S. Pat. No. 4,923,855 (1990). Jensen lists several proposed structures including chromium and nicotinic acid showing dative or coordinate covalent bonding between the nitrogen atom of the pyridine ring and also involving the carboxyl oxygen of the --COO.sup.- of the nicotinic acid. Jensen quite clearly states that the structure formed from the reaction of nicotinic acid and chromium is not known with any degree of certainty. A proposed reaction sequence is given on col . 5 and a trinicotinate structure is proposed at the top of col. 6. However, it is stated at lines 25-38 of col. 6 that it is likely that the reaction illustrated does not go to completion. It is speculated that some dinicotinate and perhaps even some mononicotinate or even pentanicotinate may be formed.
Weismann, "Chelating Drugs and Zinc", Dan. Med. Bul., 33 208-211 (1986) states that, in nature, there are various vitamin-metal complexes formed such as Zn-thiamine, Zn-pyridoxamine and Zn-biotin. Several "drugs" are also mentioned and structures are proposed for Zn-drug complexes, none of which involve vitamins. However, each structure proposed shows the conventionally described chelate bond, i.e. between the zinc ion and an amine or OH group. No .pi.-cloud or resonance bonding is taught or even suggested.
While the above mentioned complexes and chelates are of interest both from the standpoint of their biological activity and structure, they are limited in scope to certain types of activity and, for the most part, involve only the metal ions Co and Cr which are present in only very minute amounts in complexes exhibiting GTF type activity or in alleged naturally occurring Zn vitamin complexes. It would be desirous to formulate a class of chelates which would simultaneously provide the benefits of vitamins and a broad class of the minerals most commonly required in biosystems, e.g. Fe, Cu, Mn, Zn, Ca and Mg, combined in a single molecule having superior bioavailability for both mineral and vitamin.